This invention relates to pipe laying apparatus, and more particularly it is concerned with a pipe laying apparatus suitable for laying pipes of relatively small diameter.
Nowadays, a propulsion process is becoming more popular as a process for laying pipes of relatively small diameter or a diameter of less than 1000 mm than an opencut process that has hitherto been adopted. In the propulsion process, propulsion means, such as hydraulic cylinders, are installed in a starting pit for pressing pipes to be laid at a rear end thereof to propel same while forcing the earth ahead of the pipes to be compacted, to thereby lay the pipes underground. This propulsion process is specifically referred to as a compaction type system. Some disadvantages are associated with the compaction type system. That is, the pipes to be laid under ground are merely pressed at the rear thereof by the hydraulic cylinders, so that a resistance of high magnitude is offered to the movement of the pipes by a force of friction acting between the pipes and the earth through which they are propelled. This makes it necessary to use hydraulic cylinders capable of producing a pressing force of high magnitude. Also, the pipes tend to be damaged because of a force of high magnitude exerted thereon, and the pipes propelled tend to be deflected from the designed path of movement with a low degree of directional precision.
Proposals have been made to develop improved pipe laying apparatus by obviating the aforesaid disadvantages of the prior art. They include a pipe laying apparatus of a rotation excavation type as disclosed in Japanese Patent Application Laid-Open No. 29797/82, which includes an excavator equipped with a rotary excavating tool and excavates the earth by the excavator while propelling pipes to be laid by hydraulic cylinders to thereby lay the pipes underground and a pipe laying apparatus of a lateral vibration excavation type as disclosed in Japanese Patent Application Laid-Open No. 123393/82 and No. 135299/83, which incudes a vibration excavator attached to a formed end of the leading pipe and excavates the earth by the vibration excavator while propelling pipes to be laid by hydraulic cylinders, the vibration excavator having a shaft mounting eccentric weights located at right angles relative to the center axes of the pipes to be laid to produce lateral vibration with rotation of the shaft.
In the pipe laying apparatus of the rotation excavation type the earth is excavated by rotating the rotary excavating tool while a viscosity imparting liquid is being injected into the earth in the vicinity of the surface at which excavation is carried out, and at the same time the soil as excavated is mixed with the viscosity imparting liquid and agitated to produce a viscosity imparting liquid containing soil which is conveyed under pressure toward the starting pit through an annular passage defined between a hole formed by excavation and the pipes to be laid. In the pipe laying apparatus of the vibration excavation type, a forward end portion of the excavator is caused to vibrate to crush the earth into minuscule particles while a viscosity imparting liquid is being injected into the earth in the vicinity of the surface at which excavation is carried out, and at the same time the soil as excavated is mixed with the viscosity imparting liquid and agitated by the vibration of the excavator to produce a viscosity imparting liquid containing soil which is conveyed under pressure toward the starting pit through an annular passage defined between the hole formed by excavation and the pipes to be laid.
The pipe laying apparatus of the rotation excavation type can have application in the earth layers such as sand, clay, etc., but when it is a gravel layer including solid particles such gravels, the problem arises that it is difficult to convey the excavated soil toward the starting pit for discharging thereof. Meanwhile, the pipe laying apparatus of the vibration excavation type offers the advantage that even if the earth contains solid materials such as gravel, they can be embedded in the earth by the vibration while allowing only the viscosity imparting liquid containing soil of high viscosity to be discharged to the starting pit. However, this type encounters the problem that when the earth layer is clay, it is difficult to improve operation efficiency and achieve a high excavation speed unless the amplitude of vibration of the excavator is increased, while if the amplitude is increased, the vibration of the ground surface increases in magnitude.
Proposals have been made, as described in Japanese Patent Application Laid-Open No. 44194/83, for example, to use a pipe laying apparatus which possesses the merits of both the rotation excavation type and the vibration excavation type by mounting a vibrator in the excavator body having a rotary excavating tool for allowing rotation and vibration excavations to be effected, so that the apparatus can cope with a wide range of earth layers including sand, clay, gravel, etc.
Meanwhile, the pipe laying apparatus of the above-mentioned vibration excavation type also suffers the disadvantage that since the excavator body vibrates as a whole, the vibration of the excavator is transmitted to the pipes to be laid which are rigidly connected to the excavator body. Thus, the pipes to be laid are caused to vibrate simultaneously as the excavator body vibrates, and therefore it becomes necessary to increase the size of the excavator to increase the magnitude of the vibration produced.
The pipe laying apparatus of the combined rotation and vibration excavation type as disclosed in Japanese Patent Application Laid-Open No. 44194/83, noted hereinabove, has the same disadvantage as the pipe laying apparatus of the vibration excavation type since the vibrator mounted inside the excavator body causes the excavator body to vibrate as a whole.
On the other hand, vibratory excavators have been developed which are provided with means for avoiding transmission of vibration to pipes to be laid. This type of excavator is disclosed in Japanese Patent Application Laid-Open No. 146896/82, for example, which comprises a cylindrical support, a vibrator generally of a cylindrical shape and having a conical forward end portion, the cylindrical portion being coaxially inserted in the cylindrical support, a plurality of shock absorbing small-diameter rods mounted between a rearward end of the vibrator and a forward end of the support to connect them together in such a manner that the small-diameter rods are arranged in a circle concentric with the support and the cylindrical portion of the vibrator, and a bellows of resilient material mounted between the forward end of the support and a back surface of the conical forward end portion of the vibrator. The vibrator has a rotary shaft journalled in the cylindrical portion thereof coaxially therewith, and an eccentric weight is mounted on the rotary shaft which is connected to a drive motor. A pipe laying apparatus equipped with this type of vibratory excavator offers the same advantages as the first-mentioned pipe laying apparatus of the vibration excavation type of the prior art.
Moreover, the vibrator excavator noted hereinabove is equipped with shock absorbing small-diameter rods interposed between the support and vibrator. This causes a thrust applied to the pipes to be laid by hydraulic cylinders to be transmitted to the vibrator, while, the vibration of the vibrator (vibration which is at right angles to the axes of the support and the cylindrical portion of the vibrator) is absorbed by the small-diameter rods which are flexed at right angles to the axes of the support and the cylindrical portion of the vibrator, so that the vibration of the vibrator is prevented from being transmitted to the pipes to be laid through the support. Stated differently, the vibrator and the pipes to be laid are connected together in flexible coupling through the small-diameter rods and support. This makes it possible to provide improvements in the first-mentioned vibratory excavator of the prior art which suffers the disadvantage that the excavator should be large in size to develop a vibration of high magnitude due to the vibration to be transmitted to the pipes to be laid.
The problem raised with regard to this type of vibratory excavator is that the arrangement whereby a plurality of small-diameter rods are mounted between the support and vibrator to bear the thrust applied by the hydraulic cylinders has the risk that the small-diameter rods might be ruptured when the thrust applied thereto is high in magnitude.